Sewage sludge and its biochar as organic amendments: Modulating microbial activity, nutrient dynamics, and chemical properties in sandy soil cultivated with sugarcane

Abstract

Sewage sludge-derived biochar, a carbon-rich material produced by the pyrolysis of sewage sludge, has emerged as a promising amendment for enhancing the fertility and biological quality of nutrient-poor sandy soils in tropical regions. We investigated the effects of sewage sludge (SS) and its biochar (SSB) on microbial indicators, nutrient dynamics, and sugarcane biomass growth in sandy soil over 120 days. Treatments included individual applications of SS (40 Mg ha⁻¹) and SSB (20 Mg ha⁻¹), their combinations at 75:25, 50:50, and 25:75 SS:SSB ratios, a mineral fertilizer, and an unfertilized control. Microbial biomass carbon (Cmic), basal C-CO₂ flux, metabolic (qCO₂) and microbial (qMic) quotients were monitored, along with ammonium and nitrate levels, available phosphorus (P), and carbon stock. SS promoted a rapid rise in microbial activity and Cmic, whereas SSB sustained these effects over time, demonstrating complementary roles. The 75:25 combination exhibited the strongest synergistic response, enhancing microbial efficiency (higher qMic and lower qCO₂), P availability, and carbon storage. Ammonium and nitrate peaked during early and mid-stages, respectively, with the highest values under SS. At the same time, P availability and soil carbon stocks were maximized under 75:25. Sugarcane biomass increased significantly in this treatment, despite foliar N and P concentrations remaining below sufficiency levels. These results highlight clear synergistic interactions between SS and SSB, emphasizing that the 75:25 combination offers a balanced strategy to improve nutrient cycling, microbial functionality, and carbon stabilization in tropical sandy soils.